In prior industrial operations, hexane and heptane solvents have been used in the solvent extraction of oil-containing vegetable matter, for example oilseeds and oil-yielding plants. The extraction apparatus included vertical extraction towers, screw extractors and bucket extractors. Most of the equipment used in the extraction of oil-yielding vegetable material and oilseed was configured to work in a counter-current manner. With current equipment, several extraction stages are necessary in order to circulate the micella and attain sufficient wetting of the material to be extracted, thereby requiring the use of a higher proportion of solvent. In addition, overall energy consumption inherent in previous slurry separations has been excessive, if not prohibitive. An example of such a slurry separation process is disclosed in U.S. Pat. No. 2,564,409 to Rubin, issued Aug. 14, 1951.
Beyond the problem of sufficient wetting of the raw materials, another difficulty encountered in liquid slurry operations is the separation of the solvent from the extracted oil and defatted meal. Separation of normally liquid solvents from oil involves a distillation process requiring large amounts of heat. Complete removal of solvents, such as hexane, from the solids is practically impossible by conventional steam stripping techniques. Analysis of conventional soybean extractions using hexane revealed a presence of a minimum of 10% by weight of hexane in the meal after steam stripping.
Other teachings in the art have recognized the use of normally gaseous solvents at both supercritical and subcritical conditions, such as carbon dioxide and propane. For a typical solvent extraction process using propane at room temperature, the operating pressure must exceed 125 psi to remain in liquid state and even higher if temperatures are elevated. One example is described in U.S. Pat. No. 1,802,533 to Reid, issued Apr. 28, 1931, where an extraction vessel is filled with raw material at which a liquefied gaseous solvent is supplied and after extraction the micella or solvent extracted oil is distilled by heating. In U.S. Pat. No. 5,281,732 to Franke, issued Jan. 25, 1994 , the raw material is introduced into the extraction vessel, a vacuum is pulled on the vessel to exhaust any air, after which nitrogen is introduced to pressurize the extraction vessel to allow the propane solvent to remain in its liquid state.
With liquefied gaseous solvent extraction processes, the introduction of raw material into the extraction chamber as well as removal of the solid material up to this time have been complicated at high levels of pressure and has been limited to batch operations. Usually, two or more extraction vessels are operated in parallel so that one can be emptied and filled while another extraction is being carried out. This operation limits the speed and economy of the extraction to the ability of the personnel performing the exchange as opposed to a continuous process. Another limiting factor to the economy and speed of operation is the problem of maintaining the pressure in the extraction zone. During the removal of the solvent and oil, the extraction zone must be pressurized to prevent unnecessary evaporation of the solvent, which in turn, may freeze the extracted material. This “freezing” results in the operating personnel having to hammer out the frozen material and possibly causing damage to the extraction vessel. Although the material is frozen, the solvent is readily removed from the solid residue and the extracted oil due to its inherent vaporization when the pressure is reduced.
Difficult-to-extract oil-bearing plant material, such as peanuts, rice and almonds, which are preferred in the whole grain state, require a rather complicated extraction and recovery processes such as the supercritical extraction disclosed in U.S. Pat. No. 4,331,695 to Zosel, issued May 25, 1992. At supercritical temperatures and pressures, low molecular weight liquid gaseous solvents diffuse particularly easily through cell membranes, thus making it possible to extract oils and fats selectively. From 150° F. to 200° F., pressures for propane can range from 400 to 600 psi. At these conditions, the solvent selectively extracts light colored fatty matter and rejects undesirable color bodies, phosphatides and gums. When operated at supercritical conditions, the solvent used has the property of causing a separation into two phases, the light phase containing the light colored fats and oils and the heavier phase containing undesirable materials. Thus, by varying the temperature and pressure, a wide range of extraction and phase separation can be achieved. A disadvantage of supercritical extraction is the extreme pressures needed for extraction depending on the solvent used. For example, the pressure of carbon dioxide as a solvent can range from 5,000–10,000 psi. Special high pressure vessels must be designed to handle such pressure which limits the extraction process to a batch operation.
One of the more troublesome oil bearing materials is rice bran. Although rice is one of the most plentiful and nutritious food sources, it is one of the least utilized, primarily because of the difficulty in processing of the bran. Rice bran contains from about 15 to 20% oil and is not considered suitable for human consumption if high levels of Free Fatty Acids (FFA) are present in the oil. High FFA also can cause a high refining loss in processing rice bran oil (RBO). To halt the formation of FFA after milling, some bran in recent years has been stabilized with a high pressure extruder. Thus, to stabilize rice bran for the food ingredient market, many large rice mills in the United States recently have purchased extruder equipment that can be used to stabilize the bran for RBO extraction if necessary. But it has been reported that the use of such extrusion-stabilized bran results in a darker crude oil. Because of this problem, more than the normal amount of bleaching clay is required, which results in additional refining losses.
Japan is recognized as a major world leader in RBO processing, with significant long-term technical experience in processing edible grade RBO. The Japanese processors have traditionally extracted the oil within a day of milling the rice to limit the amount of FFA for economical refining of edible oil. Most of the RBO extracted in Japan is reportedly done with continuous rotocell extractors.
The quality of oil contained in freshly milled rice bran declines rapidly due to the hydrolysis of lipids that is activated in the process of milling. Hydrolysis of the lipids results in the immediate development of FFA, which can increase to as much as 10 to 15% within a day depending on temperature and humidity. Only crude rice bran oil with less than 10% FFA is considered economical to refine for edible use with conventional alkali refining. For economical recovery of edible RBO, the crude rice bran oil (CRBO) must be extracted before or immediately after milling, or the bran must be stabilized. Degumming is required immediately after extraction if the CRBO is to be stored for a long time prior to refining.
Rice and rice bran can be stabilized prior to bran removal by extractive milling of the rice in the presence of an organic solvent as described in U.S. Pat. No. 3,261,690 to Wayne, issued Jul. 19, 1966. Wayne discloses a solvent extraction process using hexane as the solvent during the milling operation, thereby removing the oil which mitigates any enzyme action on the oil. This process also uses a steam heated desolventizer commonly used in solvent extraction of cottonseed meal and soybean meal.
With respect to the extraction of oil from hydrocarbon-containing solids, the first application of solvent extraction in a refinery was the recovery of heavy lube oil base stocks by propane deasphalting. Deasphalting was developed more than forty years ago as a joint effort of Kellogg, Standard Oil Co. of New Jersey, Standard Oil Co. of Indiana and Union oil Co. of California. In the process of deasphalting oil, the selection of a solvent or solvent mixture seriously effects the economics, flexibility, and performance of the plant. The solvent must be suitable, not only for the extraction of the desired oil but also for control of the yield and quality.
Propane deasphalting has been used for several decades in the manufacture of lubricating oils and is, by far, the most selective solvent among the light hydrocarbons. At temperature ranges of 100° F. to 150° F., paraffins are completely soluble in propane while asphaltic and resinous compounds precipitate. The rejection of these compounds drastically reduce the metals and nitrogen content in the deasphalted oil and also the rejection of condensed-ring aromatics. Although deasphalting with propane has the best quality, the yield is usually less than with a heavier solvent. In order to recover more oil from vacuum-reduced crude, mainly for catalytic cracking feed, higher molecular weight solvents such as butane and even pentane have been utilized. When a unit is required to handle a variety of feedstocks, a dual or multiple solvent can provide some flexibility. For instance, a mixture of propane and n-butane would suit both heavy feed and lighter feed. By adjusting the solvent composition the desirable product quality is obtainable.
Although propane has been used successfully in extracting oils, liquid solvents have been utilized as described in U.S. Pat. No. 4,399,025 to Fletcher et al., issued Aug. 16, 1983. The Fletcher et al. process removes impurities from heavy and light lube fractions using tetrahydrofurfuryl alcohol in an extraction column. The tetrahydrofurfuryl alcohol is then removed from the extracted oil by steam distillation and stripping. Another example of liquid solvent extraction is described in U.S. Pat. No. 5,256,257 to Schiel, issued Oct. 26, 1993, which provides a continuous evaporation process for drying water-wet waste solids and sludge using a paraffin oil solvent. Schiel involves the mixing of solids and sludge with a paraffin oil solvent in multiple evaporator stages and finally separating most of the solvents from the solids by centrifugation. Both of these patents use a normally liquid solvent in combination with some type of steam heat for evaporation and separation of solvent which results in problems similar to the complete wetting and removal of the solvent associated with liquid solvent extraction of vegetable matter.
A slightly different example of a process for separating contaminants from soils and sludge is described in U.S. Pat. No. 4,977,839 to Fochtman et al., issued Dec. 18, 1990. Fochtman et al. requires the use of indirect heat from a rotary kiln or dryer, thereby subjecting the oil-containing material to temperatures effective to volatilize the contaminants with continuous removal of vapors to effect a desired degree of separation. This process has two drawbacks associated with it. First, the required high temperatures needed to volatize the contaminants into vapors is a potential fire hazard. Second, vaporizing the contaminants increases the potential release of contaminants into the air.
U.S. Pat. No. 5,066,386 to Paspek et al., issued Nov. 19, 1991, discloses a process of extracting oil from oil-water emulsions containing suspended solid particles through the use of a liquefied hydrocarbon gases at elevated pressures. In Paspek et al., the stability of oil-water-solid emulsions is a function of the composition; that is, the ratio (relative amounts) of oil, water, and solids in the mixture, as well as the type of oil and solids. The stability of the emulsion increases with the presence of suspended solids. The breaking of such emulsions requires alteration of this ratio. Removal of solids by filtration has a tendency to break the emulsion, but since the emulsion is so viscous, filtration is extremely difficult. In addition, centrifugation of oil-water solid emulsions results in rather poor separation.
Despite that teachings of the prior art, a need still exists for a process and apparatus for continuously extracting oil from oil-bearing materials which is suitable for both vegetable matter and hydrocarbon-containing solids, sludges, slurries and emulsions. Such a process and apparatus should utilize liquified normally gaseous hydrocarbon solvents at supercritical and/or subcritical temperature and pressure ranges. Such a process an apparatus also should eliminate the the problems of complete solvent removal associated with liquid slurry operations using normally liquid solvents, the inherent operating and freezing problem associated with the use of normally gaseous solvents, and the inherent operating problems associated with batch operations that use liquid normally gaseous solvents. In addition, such a process and apparatus for continuously extracting oil from oil-bearing material should provide for greater efficiency in the wetting of the oil-bearing material, should simultaneously separate solid extracted matter from the liquids, such as oils, water, and solvent, and should enable phase separation of the dense phases from the less dense phases, thereby selectively extracting and separating the light colored matter from undesirable components and eliminating desolventizer steps, toaster operations and commuting operations. Moreover, such a process and apparatus should enable grains, such as rice, almonds, soybeans, peanuts and the like to be extracted whole. Further, such a process and apparatus should allow for the extraction and milling of rice in the presence of a liquefied normally gaseous solvent, thereby eliminating the formation of free fatty acids (FFA) and the need for rice bran stabilizers and thereby selectively extracting oils lighter in color to produce a superior quality oil.